Ifstream读取函数不加载到向量 [英] Ifstream read function doesn't load into vector

问题描述

我是一个新的编程，所以我不知道如何搜索这个问题，我知道我问了其他2个问题，但我似乎无法使它工作。

I'm somewhat new to programming, so I'm not sure how to search for this problem, and I know I asked 2 other questions about this, but I can't seem to make it work.

我有一个问题，其中有一个向量：

I got a problem, where I have a vector:

vector<Device*> Devicelist_;

在这里我尝试加载设备使用此功能（我已经做了一个保存功能， ）：

Whhere I try to load Devices into using this function (I already made a Save-function, which works):

bool Devicelist::LoadFromFile() //Opdaterer vector<Device> fra fil
{
    ifstream LoadFile("Devices.dat", ios::in | ios::binary);

    if (!LoadFile)
    {
        cerr << "File could not be opened." << endl;
        return false;
    }

    LoadFile.seekg(0, ios::end);
    int numberOfDevices = LoadFile.tellg() / sizeof(Device);

    for (int i = 0; i < numberOfDevices; i++)
    {   
        Devicelist_.push_back(new Device);
        LoadFile.read(reinterpret_cast<char *>(Devicelist_[i]), sizeof(Device));
    }

    cout << Devicelist_[0]->getName() << endl;

    LoadFile.close();
    return true;
}

问题是 LoadFile.read / code>不会将任何设备加载到设备列表中。

The problem is that LoadFile.read() does not load any Devices into the devicelist.

你能看到我的问题是什么？先感谢。

Can you see what my problem is? Thanks in advance.

推荐答案

您的问题其实很简单。您忘记了重置您的获取位置：

Your problem is actually really simple. You forgot to reset your get position:

LoadFile.seekg(0, ios::end);
int numberOfDevices = LoadFile.tellg() / sizeof(Device);

for (int i = 0; i < numberOfDevices; i++)

应为

LoadFile.seekg(0, ios::end);
int numberOfDevices = LoadFile.tellg() / sizeof(Device);
LoadFile.seekg(0L, ios::beg);
for (int i = 0; i < numberOfDevices; i++)

查找数字是使用stat：

An alternative to finding the number is using stat:

#include <sys/stat.h>
int getNumberOfDevices(char *filename)
{
    struct stat st;
    return st.st_size / sizeof(Device);
}

或者，如果你想避免stat，你可以这样做：

or, if you wanted to avoid stat, you could do something like this:

bool Devicelist::LoadFromFile() //Opdaterer vector<Device> fra fil
{
    ifstream LoadFile("Devices.dat", ios::in | ios::binary);
    if (!LoadFile)
    {
        cerr << "File could not be opened." << endl;
        return false;
    }
    int numberOfDevices = 0;
    while (true)
    {   
        Device *tmp = new device; 
        LoadFile.read(reinterpret_cast<char *>(tmp), sizeof(Device));
        if (LoadFile.good()) //we successfully read one
        {
            ++numberOfDevices;
            Devicelist_.push_back(tmp);
        }
        else break; //get out of the infinite loop
    }
    cout << Devicelist_[0]->getName() << endl;
    LoadFile.close();
    return true;
}

这样，它读取所有的，而不会弄乱位置，保存计数完成后。

This way, it reads all of them, without messing around with positions, and keeps a count when it is finished.

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